When noises radiated from electronics on a vehicle enter a radio antenna and interfere with a radio broadcast, the noises are mixed with the radio sound, resulting in sounds that are offensive to the ears and uncomfortable. FIG. 6 illustrates received broadcast waves mixed with noises.
A conventionally known technique for reducing such noises is to include: an analog switching circuit that is supplied, as an input, with a sound signal carrying pulsating noises, is controlled by a control signal to turn on and off, is turned off while the pulsating noises exist, and outputs the sound signal with the pulsating noises being suppressed; and a voltage retaining circuit connected to the analog switching circuit, the voltage retaining circuit retaining, while the analog switching circuit is turned off, a voltage that exists immediately before the circuit is turned off as an interpolating voltage to replace the noises on the sound signal from the analog switching circuit (for example, see Patent Literature 1).
Another known technique is to incorporate an AM noise canceller into a multifunctional tuner IC, focusing on the performance of removing pulsating noises in a weak electric field that are most offensive to the ears during AM radio reception (for example, see Non-Patent Literature 1).
As illustrated in FIG. 5, these conventional techniques cut short-period pulsating noises from an alternator, a windshield wiper and the like that are mixed with radio, and remove the annoying noises by linear interpolation.